Among high-pressure discharge lamps, the high-pressure mercury lamp, which is used as a light source in projector-type image display devices such as liquid crystal projectors and the like, has in recent years been attracting particular attention (see Japanese Laid-open Patent Application No. H4-303592).
Generally, the high-pressure mercury lamp has a pair of opposing electrodes disposed inside an arc tube enclosing a halogen material, an inert gas, and mercury. A high-pressure discharge lamp lighting device applies a predetermined high voltage pulse to the high-pressure discharge lamp to give dielectric breakdown between the electrodes, and subsequently causes an alternating current of a predetermined frequency to flow thereby causing the lamp to light.
The life of such high-pressure mercury lamps is of the order of 2000 hours to 3000 hours.
Liquid crystal projectors in the past were used mainly in school classrooms, conference rooms, and the like, but in recent years have become increasingly popular with ordinary households.
Liquid crystal projectors use as their light source what is, with discharge being caused in the short gap between the electrodes to generate light, essentially a point light source. Consequently, image stability is strongly influenced by discharge stability. For this reason, lamp constructions and lighting methods for ensuring discharge stability in discharge lamps, such as the constructions and methods described below, have been in use for some time.
The high-pressure mercury lamps currently in use in liquid crystal projectors generally include, besides mercury, a halogen material as one of the enclosed materials. This is because the halogen cycle of the enclosed halogen material prevents the occurrence of lamp blackening due to the electrode ingredient tungsten evaporating during lighting. This halogen cycle operation is very effective in the prevention of lamp blackening. However, as the cycle continues, tungsten transferred to the tips of the electrodes via the cycle accumulates thereon, forming protuberances. The growth of such protuberances can be controlled by varying the frequency of the supplied alternating current, a method disclosed in Japanese Laid-open Patent Application No. 2001-312997 and in Japanese Laid-open Patent Application No. 2003-338394. Using this method, stability in the luminance when the lamp is dispatched is achieved by shortening the distance between the electrodes through the formation of suitable protuberances to make a point light source.
Further, growth of the protuberances during the first 100 to 500 hours or thereabouts of the life can be suppressed by varying the frequency of the supplied AC current, a method which is disclosed in Japanese Laid-open Patent Application No. 2003-338394. Using this method, the excessive growth of the protuberances generated in the first 100 to 500 hours or thereabouts of the life can be suppressed, and stability in intensity is achieved. The method involves detecting a discharge voltage and, with the voltage variation as a reference, performing feedback control to restore the tips of the electrodes.